When Israel grumbled against God in the wilderness, the LORD sent fiery serpents whose bite was lethal. The people repented and begged Moses to intercede. God commanded Moses to make a serpent of bronze and set it on a pole: "and it shall be that everyone who is bitten, when he looks at it, shall live." This is one of the most important types of Christ in the entire Old Testament, and Jesus interpreted it Himself: "As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, that whoever believes in Him should not perish but have eternal life" (John 3:14-15). The parallels are exact: (1) the Israelites were dying from serpent bites — we are dying from sin; (2) the cure was not in them but in looking at what God had lifted up — our cure is not in ourselves but in looking to the crucified Christ; (3) the bronze serpent looked like what was killing them, just as Christ "who knew no sin" was made sin for us; (4) looking was the only requirement — no works, just faith. Centuries later, Israel turned the brazen serpent into an idol and called it Nehushtan, and Hezekiah destroyed it (2 Kings 18:4) — a warning that even God-given means of grace can become idols.
Numbers 21:8-9 — "Then the LORD said to Moses, "Make a fiery serpent, and set it on a pole; and it shall be that everyone who is bitten, when he looks at it, shall live." So Moses made a bronze serpent, and put it on a pole; and so it was, if a serpent had bitten anyone, when he looked at the bronze serpent, he lived."
John 3:14-15 — "As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, that whoever believes in Him should not perish but have eternal life."
2 Kings 18:4 — "He removed the high places and broke the sacred pillars, cut down the wooden image and broke in pieces the bronze serpent that Moses had made; for until those days the children of Israel burned incense to it, and called it Nehushtan."